Objective: A central problem of ecology concerns community organization: How are resources divided among the species of an ecological community and what mutual arrangements permit the coexistence of closely related species? Interspecific interactions, involving either negative (competitive) or positive (facilitative) interactions between species, are important in forming community patterns. There is ample theoretical justification that competition might by important in community organization and dynamics, but little experimental support (MacArther and Levins 1964, Levins 1968, Miller 1969, MacArthur 1970, Wilbur 1972). Negative interactions may be most important during early community evolution before mitigation by evolved differences in resource utilization. Positive interactions, however, may be more important than negative interactions in directing day-to-day dynamic community relations. Positive interactions may also be of vital importance in determining population size either by averting an outbreak or by restoring a population after a catastrophy since they operate on a continuing basis. The dynamic nature of interspecific interactions in molding community structure and maintaining population density and structure demand further investigation. The role of interspecific interactions in community processes can be determined by considering the following questions: 1. Are interspecific interactions dynamic processes affecting interspecific relations and molding community structure or are they evolutionary relics unimportant in comtemporary community dynamics? 2. What is the effect of one species upon a coexisting species' ecology and consequently upon its methods of resource utilization? 3. What is the effect of each species upon the density and demography of other species in the community? Our initial studies have shown that interspecific competition is indeed a dynamic process in the small mammal system on the Texas Coastal Prairie. The determination of the effect of interspecific interactions upon resource utilization and population demography is important from both a theoretical and practical view. In addition to providing data concerning the role in interspecific interactions in community structure, this research will provide data to assist in predicting the outcome of artificial (man-induced) or natural perturbations (alterations) of natural systems.